1. Field of the Invention
The present invention relates to a control apparatus for a fuel vapor purge system of an internal combustion engine of a motor vehicle, which system serves to introduce or purge fuel vapor evaporated from a fuel contained in a fuel tank of the engine into an intake passage thereof so that the fuel vapor is admixed to a fuel supplied from a fuel supply system to undergo combustion within engine cylinders. More particularly, the invention is concerned with a fuel vapor purge system and a diagnosis method with a function for diagnosing the purge system as to the occurrence of a fault, and for generating an alarm upon the detection of an abnormality in the fuel vapor purge system.
2. Description of the Related Art
Heretofore, as in an attempt to prevent the fuel vapor from being discharged to the atmosphere, there have been developed and employed widely fuel vapor purge systems which are designed to purge or introduce the fuel vapor from the fuel tank into the intake passage of the engine by taking into account the operation states thereof.
FIG. 4 is a diagram which shows schematically an arrangement of a fuel vapor purge system disclosed in Japanese Unexamined Patent Publication No. 190955/1989 (JP-A-H1-190955).
Referring to the figure, an internal combustion engine 1 of a motor vehicle is equipped with an intake passage 2 for supplying a fuel air mixture to the engine. To this end, the intake passage 2 is provided with a throttle valve 3 for controlling the intake air flow and a fuel injector (not shown) for injecting fuel supplied from a fuel supply system including the fuel tank 4.
The fuel 5 is contained in a fuel tank 4 which is linked through a pipe 6 to a canister 7. Canister 7 is adapted to absorb the fuel vapor evaporated from the fuel within the fuel tank 4. The canister 7 in turn is linked to the intake passage 2 at a position downstream of the throttle valve 3 through a pipe 8 in which a purge valve 9 is installed. The purge valve 9 is controlled by a signal C outputted from an engine control unit (ECU) 10 in dependence on the engine operation state represented by signals supplied from various sensors known per se and generally denoted by a reference character D.
The pipes 6 and 8 cooperate with the canister 7 and the purge valve 9 to constitute a fuel vapor purge system for introducing or purging the fuel vapor from the fuel tank 4 into the intake passage 2.
The engine 1 is further equipped with an exhaust gas passage (not shown) and various sensors (not shown) for detecting the engine operation state. Further, the engine 1 is provided with a fuel injector for injecting the fuel 5 undergo combustion, as mentioned above. Operation timings for the fuel injector and the ignition system of the engine 1 are under the control of the engine control unit 10.
Next, a description will be made of the operation of the fuel vapor purge system shown in FIG. 4.
The engine control unit 10 controls optimally the fuel injection timing, the ignition timing and other control quantities for the engine 1 in dependence on the operation state thereof and hence the running condition of the motor vehicle. When a driver of the motor vehicle depresses the accelerator pedal, the throttle valve 3 is opened in correspondence to the extent of the depression, to thereby increase the intake air flow, whereby the amount of the fuel mixture as injected increases correspondingly.
On the other hand, the fuel vapor resulting from evaporation of the fuel contained in the fuel tank 4 is absorbed by the canister 7 through the pipe 6. The engine control unit 10 generates the purge valve control signal C in dependence on the engine operation state determined on the basis of the sensor signals D, as a result of which the purge valve 9 is opened to thereby allow the fuel vapor absorbed by the canister 7 to be sucked into the intake passage 2 through the pipe 8 under a negative suction pressure within the passage 2. Thus, the fuel vapor is supplied to the engine 1 along with the fuel mixture through the intake passage 2 to undergo combustion. In this manner, the fuel vapor is prevented from being discharged to the atmosphere.
However, when at least one of the communication pipes 6 and 8 is clogged or choked due to the occurrence of some abnormality, the fuel vapor can no longer be purged satisfactorily into the intake passage 2, resulting in an increase of the internal pressure of the fuel tank 4, which may eventually lead to the leakage of the fuel vapor from the fuel tank 4 to the atmosphere through a fuel inlet port (not shown) of the tank 4. Besides, disconnection, cracking, breakage or other faults in the pipes 6 and 8, the fuel tank 4, the canister 7 and/or the purge valve 9 will provide a cause for the leakage of the fuel vapor to the atmosphere.
In this conjunction, it must be pointed out that the driver can not perceive an abnormal situation in the fuel vapor purge system because the engine operation can be controlled ordinarily or normally notwithstanding the occurrence of such an abnormality. Consequently, the operation of the engine 1 will be continued, whereby a large amount of fuel vapor will uncontrollably be discharged to the atmosphere.
As will be appreciated from the above description, the fuel vapor purge system known heretofore suffers a problem that the fuel vapor may continue to be discharged to the atmosphere upon the occurrence of an abnormality in the system. This is because the system is not imparted with an abnormality diagnosis capability or function.